So it appears no matter what trick you use, you really can't use USERPRINCIPALNAME() on a row.

I've also tried defining USERPRINCIPALNAME() as a measure and using that but that also failed (don't recall the error right now but I'll also retry it)

I also tried using a 'standalone parameter' table to switch RLS on and off using SELECTEDVALUE but the filtered value in the parameter table never appears. ISFILTERED always returns false despite that table being filtered.